President Obama and his Western allies faced off against Russia on the international stage Tuesday, battling over how exactly to force Syria to turn over its chemical weapons, even as the president pleaded with Congress to give him time to see the diplomatic efforts through.

In an address to the nation Tuesday night, Mr. Obama said he still favors a missile strike against Syria as punishment for a chemical weapons attack. But he said he wants to wait to for the international community to work its will.

“Over the last few days we’ve seen some encouraging signs,” Mr. Obama said in a televised address from the White House. “This initiative has the potential to remove chemical weapons without the use of force.”

The president again laid out the facts of Syria’s war atrocity and said the attack cannot go unpunished.

“The facts cannot be denied,” Mr. Obama said. “The question now is what the United States of America, and the international community, is prepared to do about it.”

The president has changed his mind since Aug. 31, when he announced he wanted congressional approval to launch a limited military strike against Syria as punishment for a chemical-weapons attack ten days earlier that killed more than 1,400 people.

Early Tuesday the administration said it still wanted Congress to authorize strikes against Syria in order to strengthen the president’s negotiating hand. But by the afternoon Mr. Obama, meeting with senators, urged them to put off a vote rather than rush a resolution through that could be rejected by Congress, according to those in the closed-door meeting.

Lawmakers who were already inclined to support strikes said the threat of congressional action was critical to pushing Syria and Russia to the negotiating table, and said passing a resolution authorizing strikes could indeed bolster the president’s hand.

Those who have opposed strikes, however, said approving a war plan now could undercut the diplomatic efforts, which they said were far preferable to a unilateral strike that could leave the U.S. enmeshed in the two-year-old Syrian civil war.

Russian President Vladimir Putin echoed that same message, telling the Russian television network RT that Mr. Obama must withdraw his threat of force against Damascus in order to complete an international deal.

“Certainly, this is all reasonable, it will function and will work out, only if the U.S. and those who support it on this issue pledge to renounce the use of force, because it is difficult to make any country — Syria or any other country in the world — to unilaterally disarm if there is military action against it under consideration,” Mr. Putin told RT.

Syrian officials, meanwhile, signaled they would be willing to accept the Russian proposal, which calls for the regime of Syrian President Bashar Assad to turn its chemical weapons over to an international authority and to sign onto the international treaty banning the use of chemical weapons. Syria is one of just five countries never to have signed the Chemical Weapons Convention barring the use of chemical weapons.

Russia’s surprise offer Monday upended the debate both in Washington and at the U.N., where France and Russia were both working on versions of an agreement.

France’s version would have included stiff penalties if the deal were broken, but the Russian government objected to that and was working on an alternative. The U.N. Security Council scheduled, then canceled, a meeting to talk about a path forward.

In Washington the State Department, which on Monday had distanced itself from the proposal, sought to take credit on Tuesday, insisting that Secretary of State John F. Kerry had talked with Russian Foreign Minister Sergey Lavrov about it beforehand.

The State Department announced Mr. Kerry will fly to Geneva to meet with Mr. Lavrov Thursday to see if they can finalize a deal.

Sidetracked

The latest developments also sidetracked Congress, where senators had been barreling toward an early test vote Wednesday but have now put that off indefinitely to see how the latest bout of international diplomacy plays out.

Senate Majority Leader Harry Reid even canceled a planned all-senators’ briefing to give the administration time to pursue the latest options.

“There are a lot of things going on and I think the briefing tomorrow would be premature with all the moving targets,” he said as he closed the Senate down Tuesday evening.

Early in the day top Obama administration officials pleaded for Congress to still act, saying it would bolster the president’s negotiating abilities with Russia and Syria.

“The president believes we need to keep this threat, this reality, absolutely on the table,” Mr. Kerry told the House Armed Services Committee. “He wants the Congress to act.”

But hours later Mr. Obama, who met behind closed doors on Capitol Hill first with Senate Democrats and then with Senate Republicans to make his case for strikes, said he would accept a delay — as long as Congress doesn’t outright vote down his authority to use force.

“The basic message was this: Keep the threat of a military response open,” said Sen. Tom Carper, Delaware Democrat. “If you do that, you’ll have a much better chance of the Syrians and the Russians actually doing what they’ve been talking about. If we don’t keep that threat open, they may very well walk away.”

Last week a resolution authorizing military strikes cleared the Senate Foreign Relations Committee on a 10-7 vote, but on Tuesday a group of senators was working to rewrite the language to incorporate the potential Russian deal.

Senators said they wanted to give the Assad regime a 45-day window to sign the international convention against chemical weapons and turn over its existing stockpile, and would empower Mr. Obama to strike at the end of that time. Under that resolution, he would not need to come back to Congress for a second vote.

“If the government of Syria does not sign and comply with the convention within 45 days after the date of the enactment of this resolution, all elements of national power will be considered by the United States government,” said Sen. Joe Manchin, a West Virginia Democrat who is at the center of negotiations. “So he has a right to consider everything and every force that he needs.”

National appeal

In his evening address Mr. Obama said America isn’t the world’s policeman and added that he has “resisted calls for military action because we cannot resolve someone else’s civil war through force.”

But he said the situation changed on Aug. 21 when Syria launched its attack near Damascus.

“The images from this massacre are sickening,” Mr. Obama said. “We know the Assad regime was responsible. When dictators commit atrocities, they depend upon the world to look the other way.”

But the president and his advisers are still having a tough time making their case.

Early Tuesday Sen. Mitch McConnell, the Republicans’ floor leader, became the first of the top four congressional leaders to oppose strikes.

“This proposal just does not stand up,” he said on the Senate floor. “Our vital national security risks are clearly not in play.”

The Kentucky Republican said it will take more than just military strikes for Mr. Obama to regain world leadership.

“This one punitive strike we’re debating could not make up for the president’s performance over the past five years,” Mr. McConnell said. “The only way — the only way — for him to achieve the credibility he seeks is by embracing the kind of serious, integrated national security plan that matches strategy to resources, capabilities to commitments, and which shows our allies around the world that the U.S. is fully engaged and ready to act at a moment’s notice in all the major areas of concern around the globe.”

In opposing strikes, Mr. McConnell puts himself in line with most voters and with most GOP lawmakers in Washington, who are either leaning against or have announced opposition to an attack.

But House Speaker John A. Boehner said he remains behind Mr. Obama.

“I’ve supported every president that I’ve served under for the last 23 years when it comes to the use of military force,” Mr. Boehner said. “There’s one person who speaks for the United States of America when it comes to foreign policy, and that’s the president of the United States.”